The family home in a densely populated slum called Janta Colony on the outskirts of India's sprawling capital Delhi, was about the size of a typical Australian bedroom. They had two mattresses to sleep on that doubled as a "lounge" to sit on during the day. Meals were eaten sitting on the floor and they shared a tiny squat toilet with their landlords. The familys possessions were kept in a few steel trunks.

Open drains ran along the narrow alleys near the house and there was a putrid canal not far from the front door.

Mark Delaney with his son Tom near their Delhi home in 2009.

Photo: Brendan Esposito

But the Delaney family werent complaining. They spoke powerfully about the fulfilment the family had found living among the poor.

At the time Cathy Delaney, who holds a masters degree in mathematics, told me life in the slum had made her “freer” of money and possessions.

“These things don't define my life,” she said.

Mark, who trained as a lawyer, admits his move from first-world professional to slum dweller was a “career path from hell”. But he too described how the family had been enriched by their "radical detox" from consumer society.

Slum life has nurtured a strong sense of social justice in the Delaney boys.

Tom, who was 12 years old when I interviewed the family in 2009, told me the experiences had made him “realise the most important thing is to help other people”.

Part of the slum in Delhi where Mark Delaney and his family lived.

Photo: Brendan Esposito

Another striking feature of the Delaneys' slum lifestyle was their small environmental footprint. They used very little electricity, created a small amount of waste and relied exclusively on public transport.

Like many Indian slum families, the Delaneys have often had to move. One Delhi shanty town they lived in was completely demolished and the residents scattered to “resettlement areas” on the urban fringe.

In all, the Delaney family has lived in slums for more than 13 of the past 23 years.

During that time they have became increasingly concerned about the effects of climate change.

When the family returned to Australia for an extended period in 2014 they were shocked at how unconcerned most people were about the effects of global warming.

“We expected everyone would be taking this very seriously when we got back to Australia,” says Mark. “But we were just plain wrong. People didnt even seem to want to talk about it.”

He felt like the “boy in the story of the emperor with no clothes” on his return to comfortable suburbia. “Weve seen something for what it is but most people seem to be walking around willingly blind,” he said.

“People seem to think that if responding to climate change affects my lifestyle then Im just not interested'.”

Mark Delaney returned to Australia earlier this year.

Photo: Louise Kennerley

He is especially worried that economic progress made recently in developing countries like India could be negated by climate change.

“All the development work that so many people have done for decades could be undone,” he said.

The family has always sought to serve – and learn from – their impoverished neighbours.

“We believe life isnt just about earning money and getting comfortable, but rather about attempting to make the world a better place,” Mark and Tom write in the book.

Once the family decided to live on a similar income to their neighbours and cut their monthly budget from 10,000 rupees (equivalent to about $310 at the time) to 5000 rupees.

“First we ran out of cornflakes, and then we ran out of jam, so our diet got a lot simpler” says Mark. “But it gave us so much respect for those around us.”

As the boys grew older they developed a deep interest in climate science.

The books co-author Tom Delaney has been “a driver” in the familys radical commitment to a low carbon lifestyle.

“Tom is a real science head and he loved researching it,” says father Mark.

“Even as a young bloke he kept coming to us and saying look what I found [about climate science]. The more weve read, the more we feel this is the issue of our generation. If we dont nail this were in all sorts of trouble.”

One early response the family made to global warming was to become vegetarians, in part because of the high carbon emissions of commercial meat production. It was not an easy choice in slums where most of their neighbours were meat-eating Muslims.

In the book Mark describes how he trialled ways to reduce his milk intake because of the carbon emissions created by large-scale dairy production. “I decided to use soy milk where feasible, but not be too strict on myself,” he writes.

The family has also experimented with low-carbon travel. When the family travelled from Delhi to Brisbane in 2014 they carefully researched ways to limit the carbon emissions from the journey.

The first step was a 36-hour train trip to the city of Chennai in south India. They then took two medium-length flights to Kuala Lumpur then to Darwin (the Delaneys' research showed these flight lengths were the most carbon efficient). Finally they travelled overland from Darwin to Brisbane.

“The journey was about three times more expensive and seven times as longer than a direct flight but worked out the carbon footprint was a third to a half,” says Mark. “The key was to reduce the miles flown.”

During the past few months Mark has travelled from Brisbane to Sydney and Melbourne to promote the new book but all the interstate transport has been by train – the lowest carbon alternative.

“We wanted to have integrity writing the book,” he says.

The book draws on the familys deep engagement with poor communities in India.

The authors compare the carbon footprint of an average Australian – called “Bruce” – with that of Rukshana, one of the Delaneys real-life Indian slum neighbours. The contrast neatly illustrates the vast gulf in lifestyles and draws attention to the challenges poor young women like Rukshana face in urban India. It also reveals her annual carbon emissions are a tenth of the Australian average.

Public debate and media reporting on climate change in Australia focuses heavily on government policies and political conflict far removed from daily life and personal behaviour. Mark and Tom Delaneys book shifts attention to the behaviour of individuals and what do they can do to reduce their own carbon emissions.

They urge readers to experiment with ways to trim their carbon footprint.

“At the moment each Australian produces about 23 tonnes of carbon dioxide equivalent a year but well have to get down to two to three tonnes,” says Mark.

“What we are saying is dont look at the very low goal bit, just look at the next step you are going to take. How can you reduce that a bit over the next year or so.”

The book recommends a “spirit of positive experimentation” when trying to reduce carbon emissions.

“Just try something for a little while, give it a shot and see what its like,” says Mark. You might find its a really positive thing. It can actually be enriching for ordinary people like you and me to live low-carbon lives.”

“We are asking people to just take the next step. We are asking people to experiment.”

The Delaneys experimented with a low-carbon lifestyle when they lived in Australian during 2016. At the end of the year they estimated their annual carbon emissions per person that year were about 4.5 carbon dioxide equivalent tonnes.

“We still had some improvements to make,” says Mark.

The Delaneys have reached out to schools in different parts of Australia and offered to speak about their experience living in slums and ways to respond to climate change. But the reception has been lukewarm.

One Melbourne school recently invited Mark to speak about his life in India but not about climate change.

“Thats typical of the response – by and large people are just not really interested,” he says.